GENEVA -- More than 140 nations adopted the first legally-binding international treaty Saturday aimed at reducing mercury emissions, capping four years of negotiations on how to set limits on the use of a highly toxic metal.

The treaty was adopted after all-night negotiations that capped a week of talks in Geneva, UN environmental officials and diplomats said. A signing ceremony will be held later this year in Japan, and then 50 nations must ratify it before it comes into force, which officials said they would expect to happen within about three to four years.

"To agree on global targets is not easy to do," UNEP executive director Achim Steiner told a news conference. "There was no delegation here that wished to leave Geneva without drafting a treaty."

The treaty will for the first time set enforceable limits on emissions of mercury, which is widely used in chemical production and small-scale mining, and to exclude, phase out or restrict some products that contain mercury.

"We have closed a chapter on a journey that has taken four years of often intense, but ultimately successful negotiations and opened a new chapter towards a sustainable future," said Fernando Lugris, the Uruguayan diplomat who chaired the negotiations.

But some supporters of a new mercury treaty said they were not satisfied with the agreement.

Joe DiGangi, a science adviser with advocacy group IPEN, said that while the treaty is "a first step," it is not tough enough to achieve its aim of reducing overall emissions.

For example, he said, there is no requirement that each country create a national plan for how it will reduce mercury emissions.

But proponents of the treaty say it will set meaningful controls and reductions on a range of products, processes and industries where mercury is used, released or emitted.

These would include medical equipment like thermometers, energy-saving light bulbs, mining and cement and coal-fired power industries.

Swiss environmental ambassador Franz Perrez said the treaty "will help us to protect human health and the environment all over the world."